Marine streamer having variable stiffness

ABSTRACT

Disclosed are methods and systems for performing marine geophysical surveys that utilize a streamer having variable stiffness. An embodiment discloses a sensor streamer comprising: an outer surface; tension members within the outer surface extending along a length of the sensor streamer; spacers disposed within the outer surface along the length of the sensor streamer; a geophysical sensor disposed in an interior of one of the spacers; and an actuator assembly configured to apply tension to the tension members.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/950,005, entitled “Marine Streamer Having Variable Stiffness,” filedon Jul. 24, 2013, which claims the benefit of U.S. ProvisionalApplication No. 61/774,948, entitled “Marine Streamer Having VariableStiffness,” filed on Mar. 8, 2013, the entire disclosures of which areincorporated herein by reference.

BACKGROUND

The present invention relates generally to the field of marinegeophysical surveying. More particularly, in one or more embodiments,this invention relates to methods and systems for performing marinegeophysical surveys that utilize a streamer having variable stiffness.

Techniques for marine surveying include marine geophysical surveying,such as seismic surveying and electromagnetic surveying, in whichgeophysical data may be collected from below the Earth's surface.Geophysical surveying has applications in mineral and energy explorationand production to help identify locations of hydrocarbon-bearingformations. Certain types of marine geophysical surveying, such asseismic or electromagnetic surveying, may include towing an energysource at a selected depth—typically above the seafloor—in a body ofwater. One or more geophysical sensor streamers also may be towed in thewater at selected depths by the same or a different vessel. Thestreamers are typically cables that include a plurality of geophysicalsensors disposed thereon at spaced apart locations along the length ofthe cable. The geophysical sensors may be configured to generate asignal that is related to a parameter being measured by the sensor. Atselected times, the energy source may be actuated to generate, forexample, seismic or electromagnetic (“EM”) energy that travelsdownwardly into the subsurface rock. Energy that interacts withinterfaces, generally at the boundaries between layers of rockformations, may be returned toward the surface and detected by thegeophysical sensors on the streamers. The detected energy may be used toinfer certain properties of the subsurface rock, such as structure,mineral composition and fluid content, thereby providing informationuseful in the recovery of hydrocarbons.

In geophysical surveying, the streamer is typically a cable that isstored on a drum on the towing vessel. The streamers are typically madeof multiple components, such as electrical conductors, fiber optics, andstress-supporting members, all bundled together and covered with aprotective outer skin. The streamer may be up to several kilometers inlength. In general, the streamer has little stiffness in directionsother than inline, so it can move easily both laterally and intorsion/rotation when deployed in the water. When sensors such asvelocity, position, and acceleration sensors are incorporated into thestreamer, the movements are picked up directly by the sensors. Unlikehydrophones which only pick up the movements indirectly because ofimprovements over the years, these other sensors may have a high levelof noise which is not interesting for the marine survey. For example,the noise may be measurements of local conditions in the surroundingwater rather than reflections from the Earth below.

Under a load of pressure on only a small portion of the outside, astreamer will bend, held back only by the tension, bending and torsionalstiffness of the streamer, and the mass of the cable content, dependingon the direction, distribution, and size of the pressure. Low bendingand torsional stiffness for the streamer should result in little addedmass, but cause large local movement. As result, the streamer may havelarge local sensor recordings (i.e., measurements of local conditions inthe surrounding water) and also large waves of motion traveling throughthe cable. Low stiffness may also result in large sagging of the cablebetween steering devices with wings commonly used to control lateral orvertical position of the streamer. Because of the large sagging, theremay be large angles between the cable and fluid flow, further increasingturbulence and noise generation from hydrodynamic flow.

Accordingly, there is a need for improved methods and systems fordeploying streamers in seismic surveys having increased stiffness bothlaterally and in torsion to reduce noise sources in marine seismicsurveys.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention and should not be used to limit or define theinvention.

FIG. 1 illustrates an example embodiment of a marine geophysical surveysystem that comprises a streamer having variable stiffness in accordancewith the present invention.

FIG. 2 illustrates a cut-away view of an example embodiment of astreamer segment in a storable configuration in accordance with thepresent invention.

FIG. 3 illustrates a cut-away view of one end of the streamer segment ofFIG. 2.

FIG. 4 illustrates a cut-away view of the opposite end of the streamersegment of FIG. 2.

FIG. 5 illustrates a cut-away view of an example embodiment the streamersegment of FIG. 2 in an operational configuration in accordance withembodiments of the present invention.

FIG. 6 illustrates a cut-away view of one end of the streamer segment ofFIG. 5.

FIG. 7 illustrates a cut-away view of an example embodiment of a spacerin accordance with the present invention.

FIG. 8 illustrates a cut-away view of another example embodiment of aspacer in accordance with the present invention.

FIG. 9 illustrates an end view of an example embodiment of the spacer ofFIG. 8.

FIG. 10 illustrates a side view of an example embodiment showing tensionmembers running between spacers in accordance with embodiments of thepresent invention.

FIG. 11 illustrates an assembly of streamer segments 60 in accordancewith embodiments of the present invention.

DETAILED DESCRIPTION

The present invention relates generally to the field of marinegeophysical surveying. More particularly, in one or more embodiments,this invention relates to methods and systems for performing marinegeophysical surveys that utilize a streamer having variable stiffness.Embodiments may include the streamer having a storable configuration inwhich the streamer may be deployed and stored on a drum onboard a surveyvessel, for example. Embodiments may further include the streamer havingan operational configuration in which the streamer has a higherstiffness than in the storable configuration. After deployment into thewater, for example, the streamer may be changed from the storableconfiguration to the operational configuration. Some or all of thegeophysical survey may then be conducted with the streamer in theoperational configuration.

FIG. 1 illustrates a marine geophysical survey system 5 in accordancewith embodiments of the present invention. In the illustratedembodiment, the system 5 may include a survey vessel 10 that moves alongthe surface of a body of water 15, such as a lake or ocean. The surveyvessel 10 may include equipment, shown generally at 20 and collectivelyreferred to herein as a “recording system.” By way of example, therecording system 20 may include one or more devices (none shownseparately) for determining geodetic position of the survey vessel 10(e.g., a global positioning system satellite receiver signal), detectingand making a time indexed record of signals generated by each of aplurality of geophysical sensors 25, and/or for actuating one or moreenergy sources 30 at selected times.

The survey vessel 10 or a different vessel (not shown) may tow a sourcecable 35 that includes the one or more energy sources 30. In otherembodiments (not shown), one or more of the energy sources 30 may bemounted to the hull of the survey vessel 10. The energy sources 30 maybe any selectively actuable sources suitable for subsurface geophysicalsurveying, including without limitation, electromagnetic fieldgenerators, seismic air guns, water guns, marine vibrators or arrays ofsuch devices.

The survey vessel 10 or a different vessel (not shown) may also tow astreamer 40 through the body of water 15. As will be discussed in moredetail below, the streamer 40 may have a variable stiffness inaccordance with embodiments of the present invention. For example, thestreamer 40 may have a storable configuration in which the streamer 40has a stiffness that allows the streamer 40 to be wound onto a winch andstored on the survey vessel 10. At a desired time, the streamer 40 canbe rigidified such that the streamer 40 is changed to an operationalconfiguration having a higher stiffness than in the storableconfiguration. For example, the bending, inline, and/or torsionalstiffness of the streamer 40 may be increased. The geophysical sensors25 may be disposed on the streamer 40 at spaced apart locations. Thetype of the geophysical sensors 25 is not a limit on the scope of thepresent invention and may be particle motion-responsive geophysicalsensors such as geophones and accelerometers, pressure-responsivegeophysical sensors such as hydrophones, pressure timegradient-responsive geophysical sensors, electrodes, magnetometers,temperature sensors or combinations of the foregoing. A lead-in 45 maycouple the streamer 40 to the survey vessel 10. In the illustratedembodiment, the lead-in 45 may comprise a cable. In some embodiments,the streamer 40 may be towed near the surface of the body of water 15,for example, at a depth of about 25 meters or less, for example. Inalternative embodiments, the streamer 40 may be towed at a deeper depth.For example, the streamer 40 may be towed at a depth of up to about 50meters or more. While the present example, shows only one streamer 40,the prevent invention is applicable to any number of laterally spacedapart streamers towed by the survey vessel 10 or any other vessel. Forexample, in some embodiments, eight or more laterally spaced apartstreamers may be towed by the survey vessel 10, while in otherembodiments, up to twenty-six or more laterally spaced apart streamersmay be towed by the survey vessel 10. Although shown with a nearlyhorizontal depth profile, streamer 40 may have a variable depth profilewhen deployed in the water.

During operation, certain equipment (not shown separately) in therecording system 20 may actuate the one or more energy sources 30 atselected times. In seismic surveying, actuation of the energy sources 30should cause seismic energy to emit from the energy sources 30 with aseismic signal propagating downwardly through the body of water 15 andinto one or more rock formations 50 below the water bottom 55. Amodified seismic signal that is reflected by the rock formations 50 maybe detected by the geophysical sensors 25 as the modified signal travelsupwardly through the body of water 15, for example. In electromagneticsurveying, actuation of the energy sources 30 should generate electricand/or magnetic fields in the water 15 that interact with the rockformations 50. The electric and/or magnetic fields can be detected bythe geophysical sensors 25. The detected signal and/or fields may beused to infer certain properties of the rock formations 50, such asstructure, mineral composition and fluid content, thereby providinginformation useful in the recovery of hydrocarbons, for example.

A streamer 40 as shown in FIG. 1 may be made from a plurality ofstreamer segments 60 connected end-to-end behind the survey vessel 10(shown on FIG. 1). FIGS. 2-6 illustrate views of a streamer segment 60having a variable stiffness in accordance with example embodiments ofthe present invention. FIGS. 2-4 illustrate the streamer segment 60 in astorable configuration. FIGS. 5 and 6 illustrate the streamer segment 60in an operational configuration in which the streamer segment 60 has anincreased stiffness, such as bending, inline, and/or torsionalstiffness. In addition, the streamer segments 60 may also be used toform the source cable 35 in embodiments of the present invention. Thestreamer segments 60 may be a structure for a number of items, includingfeed lines, gas lines, optical and/or electrical signals, power,external devices, geophysical sensors, tension sensors, and geophysicalsources. The streamer segment 60 may have a length, for example, in arange of from about 10 meters to about 150 meters and, alternatively,from about 60 meters to about 150 meters. Depending on the particularapplication, the streamer 40 formed from the streamer segments 60 mayhave a length in a range of from about 200 meters to about 2000 metersor longer, for example. In some embodiments, the streamer segments 60may each have an aspect ratio (ratio of width to height) of about 1 toabout 8, for example. The width of the streamer segments 60 generallyrefers to a measurement of the extent of a particular streamer segment60 from one side of the streamer segment 60 to the other while theheight refers to the measurement of the extent of the particularstreamer segment 60 from bottom of the streamer segment 60 to its top.For streamer segments 60 that are generally circular in cross-section,the width and height should both be equal to the diameter so that theaspect ratio should be approximately 1.

In some embodiments, the streamer segment 60 may have a proximal (i.e.,close to the survey vessel 10) end 65 and a distal (i.e., far from thesurvey vessel 10) end 70. In the illustrated embodiment, the streamersegment 60 includes an outer surface (such as jacket 75), spacers 80,tension members 85, and a tensioning actuator 90. In some embodiments,the outer surface may be identified by a jacket 75 which at leastpartially covers streamer segment 60. The jacket 75 generally mayfunction as a partial or complete exterior cover that protects theinternal components of the streamer 60 from water intrusion, forexample. In some embodiments, the jacket 75 may be made from a flexible,acoustically transparent material, which may be a plastic and/orelastomeric material, such as polyurethane. One or more plates may belocated at or near the axial end of the jacket 75. For example, aproximal plate 95 may be located at or near the proximal end 65, and adistal plate 100 may be located at or near the distal end 70.

In some embodiments, the tension members 85 may extend generally theentire length of streamer 40. In general, the tension members 85 mayfunction to provide the streamer segment 60 with the ability to carryaxial mechanical load, for example. For example, the tension members 85may carry axial load along the length of the streamer segment 60. Insome embodiments, the tension members 85 may be a metal, such as steel(e.g., stainless steel) or high strength plastic materials. Examples ofsuitable plastic materials include aramid fibers such as Kevlarpolyamides (also referred to as “aramid fibers”) such as (e.g., aramids,such as Kevlar, Pol. The tensions members 85 may be in the form of acable or fiber robe, for example. At the proximal end 65, the tensionmembers 85 may extend proximally beyond the proximal plate 95. Thetension members 85 may also extend proximally beyond an actuator plate105 which is located at the proximal end 65. As illustrated, from theproximal end 65, the tension members 85 may extend through the actuatorplate 105, through the proximate plate 95, and then through the spacers80 to the distal end 70. At the distal end 70, the tension members 85may extend through the distal plate 100. In accordance with presentembodiments, tension members 85 can translate axially with respect tothe actuator plate 105, the proximal plate 95, the spacers 80, and thedistal plate 100. For example, in the storable configuration of thestreamer segment 60, the tension members 85 may be allowed to slide inthe spacers 80. Nuts (such as axial nuts 110 and distal nuts 115) orother suitable mechanical stops may be located at either axial end ofthe tension members 85. As illustrated, the axial end of the tensionmembers 85 may be spring-loaded with springs 120 disposed on thetensions members 85 between the distal nuts 115 and the distal plate100. The springs 120 may be compressed as tension is applied to thetension members 85 via the tensioning actuator 90.

As illustrated, the streamer segment 60 may further comprise a pluralityof spacers 80 disposed along the length of the streamer 40. As will bediscussed in more detail below, the spacers 80 may be so densely packedthat, when the tensioning actuator 90 applies tension to the tensionmembers 85, the spacers 80 may be compressed together aligning thespacers 80 in a rigid line. The spacers 80 may be made from a foammaterial to provide buoyancy, for example. For example, the spacers 80may include a foamed material that fills void spaces (e.g., foamedmaterial 140 on FIGS. 7 and 8), such as a foamed polyurethane or othersuitable material. As illustrated, a large volume of the streamersegment 60 may be occupied by the spacers 80. For example, at leastabout 50% of the internal volume and as much as 90% or more of theinternal volume of the streamer segment 60 and/or streamer 40 may beoccupied by the spacers 80. Typically, oil or other suitablevoid-filling material occupies the interior volume of the streamersegment 60. However, because a high volume of the streamer segment 60may be occupied by the spacers 80, less oil or other void-fillingmaterial can be used, thus minimizing potential problems that may becaused by leakage. In addition, foamed materials such as rigid foams candeal with the pressures of more than a few meters while also giving morethan 4 times the buoyancy of some void-filling materials, such as oils.Moreover, the interior of the spacers 80 may be formed with closed,hollow cavities so that a leak in the jacket 75, for example, would notnecessarily fill the entire volume.

In some embodiments, the tensioning actuator 90 may be located at theproximal end 65 of the streamer segment 60. The tensioning actuator 90may generally be configured to apply tension other than towing tensionto the tension members 85. As illustrated, the tensioning actuator 90may be coupled to the actuator plate 105. The tensioning actuator 90 maycause the actuator plate 105 to move axially outward into engagementwith the proximal bolts 110 or other mechanical stop on the tensionmembers 85. The actuator plate 105 transfers mechanical force fromtensioning actuator 90 to the tension members 85 as will be discussed inmore detail below. One example of a suitable tensioning actuator 90 is alinear drive that generates motion in a straight line to move theactuator plate 105. Other suitable actuated drives may also be used forapplying tension to the tension members 85 in accordance withembodiments of the present invention. Some examples of suitable actuateddrives may be electric or mechanical (e.g., hydraulic, pneumatic)actuated drives.

Those of ordinary skill in the art, with the benefit of this disclosure,should appreciate that other components, sensors, actuators,transducers, conductor cables, and other electronics (e.g., tanks,batteries, etc.) may also be incorporated into the streamer segments 60.Example sensors (e.g., geophysical sensor 25 on FIG. 1) that may beincorporated include sound/pressure sensors, motion sensors (speed,velocity, and/or acceleration), EM sensors, magnetism (e.g., compass),pressure sensors, depth sensors, tilt sensors, tension sensors, surfaceor bottom echosounders/mappers, among others. In some embodiments, oneor more actuators may be incorporated into the streamer segments 65.Example actuators may include control surfaces, ballast tanks, openings,covers/lids, and connection points, among others. For example, controlsurfaces (such as wings) for steering or rotational position may beused. The control surfaces may act to provide depth and/or lateralcontrol for the streamer segments 60. Moreover, the control surfaces mayallow the streamer segments 60 to perform a desired move while in thewater, such as an undulation, surfacing, diving, rescue, or recovery.Ballast tanks may be also be incorporated that can allow the streamersegments 60 to maintain depth, surface, or compensate for waterintrusion, such as by gassing a flooded chamber in the streamer segments60. Openings may also be provided for access to sensor surfaces,ballast, and/or weight/mass center manipulation. Connection points thatare openable and/or closable may also be provided in the streamersegments 60, such as valves or ports for feed or transmission lines.Covers/lids that are openable and/or closable may also be provided,which may enable cleaning and/or streamlined handling, for example.Conductor cables that may be incorporated into the streamer segments 60may include insulated electrical conductors and/or optical conductorsfor conducting electrical and/or optical signals to/from the recordingsystem 20 shown on FIG. 1. In some embodiments, one or more of theconductor cables may also carry electrical power to various processingcircuits disposed in the streamer segment 60, for example.

In accordance with present embodiments, the streamer segment 60 may havea variable stiffness. For example, the streamer segment 60 may have astorable configuration in which the streamer may be deployed and storedon a drum onboard a survey vessel (e.g., survey vessel 10 on FIG. 1).FIGS. 2-4 illustrate the streamer segment 60 in the storableconfiguration. In the storable configuration, the tensions members 85may be allowed to slide with respect to the spacers 80. The spacers 80may be packed closely in the streamer segment 60, but have enough gapsin between for the streamer segment 60 to bend. As best seen in FIG. 2,the streamer segment 60 may freely bend in the storable configuration,for example, to wound onto a drum. At a desired time, the streamersegment 60 may be placed in an operational configuration in which thestreamer segment 60 has an increased stiffness. In the operationalconfiguration, the streamer segment 60 is generally characterized asbeing rigid in that it has as bending, torsion, and/or inline stiffnessthan can be maintained for considerable lengths, for example, up toabout 10 meters, about 50 meters, about 100 meters, or even longer whenassembled end-to-end with other streamer segments 60 in the operationalconfiguration. Unlike cables and structures that have been usedpreviously as lead-ins and streamers, the streamer segments 60 in theoperational configuration will generally not form catenary, sinushyperbolic, or parabolic curvatures over at least portions of thelength, but will rather generally exhibit elastic behavior withdeformation according to deformation of beams. In some embodiments, anassembly of streamer segments 60 may be characterized as being rigid fora length of about 25 meters or longer wherein the streamer segments 60have a smallest width or height of about 1 meter or less.

Embodiments of the present technique may be used with a streamersegments 60 having a bending stiffness of 700 Newton-square meters(“Nm²”) or greater when in the operational configuration. In someembodiments, the streamer segment 60 or chain of streamer segments inthe operational configuration may have a bending stiffness of 700Newton-square meters (“Nm²”) or greater for considerable lengths (e.g.,over about 25 meters or more). The stiffness of 700 Nm² corresponds to astiffness in a cantilever beam of 1-meter length fixed in one end with aload of 1 Newton in the other, deforming roughly 0.5 mm under the load.This corresponds to an aluminum (with Young's modulus of 70 GPa) tubewith a 2-inch outer diameter and a thickness of 0.2 millimeters, a steel(with Young's modulus of 210 GPa) tube with a 2-inch outer diameter witha thickness of 0.03 millimeters or a circular rod with a Young's modulusof 2 GPa. Each of these items, i.e., the aluminum tube, the steel tube,and the circular rod, are examples of items with a bending stiffness of700 Nm². A 2-inch outer diameter typically requires 5% deformation to bewound on a 2-meter drum, which is difficult for most materials. Mostrigid materials can deform a maximum of 0.1% or, in extreme cases, 1% sothey cannot be wound on a drum without being wound in a wire orumbilical. Lower strength materials may be able to deform but will thenbe soft to enable bending. In the operational configuration, thestreamer segment 60 may be in danger of damage or permanent deformationif subjected to 3 kN or more.

FIGS. 5 and 6 illustrate the streamer segment 60 in the operationalconfiguration. In some embodiments, the spacers 80 may be so denselypacked that, when the tensioning actuator 90 applies tension to thetension members 85, the spacers 80 may be compressed together aligningthe spacers 80 in a rigid line, as best seen in FIG. 5, increasing thebending stiffness of the streamer segment 60. The spacers 80 incompression and the tension members 85 in compression should contributedto the bending stiffness of the streamer segment 60.

In the illustrated embodiment, the tensioning actuator 90 may be used toapply tension to the tension members 85 for placement of the streamersegment 60 into the operational configuration. As best seen by comparingFIGS. 3 and 6, the tensioning actuator 90 may cause the actuator plate105 to move axially outward into engagement with the proximal bolts 110or other mechanical stop on the tension members 85. In this manner, theactuator plate 105 may transfer mechanical force from tensioningactuator 90 to the tension members 85 causing the tension members 85 tomove axially. As the tension members 85 move, the distal nuts 115 on thetensions members 85 engage the distal plate 100 causing the distal plate100 to apply force onto the spacers 80 compressing the spacers 80between the distal plate 100 and the axial plate 95. Compression of thespacers 80 aligns the spacers 80 into a rigid line.

In some embodiments, the tension applied to the tension members 85 bythe tensioning actuator 90 may be higher than operational tension fromtowing. When this operational tension is added to the tension from thetensioning actuator 90, the load increases on the tension members 85while decreasing on the spacers 80. If this operational tension isgreater than the tension from the tensioning actuator 90, the spacers 80may decompress leaving some gaps that could reduce stiffness of thestreamer segment 60. Accordingly, it may be desired in some embodimentsfor tension from the tensioning actuator 90 to exceed the operationaltension from towing. When in tension from the tensioning actuator 90,all the spacers 90 should move when torsion of bending loads are addedto the streamer segment 60 and, thus, should add stiffness according totheir moment of inertia times their Young's modulus.

FIG. 7 illustrates a spacer 80 a that can be incorporated into thestreamer segment 60 shown on FIGS. 2-6 in accordance with embodiments ofthe present invention. As illustrated, the spacer 80 a may have aprotective outer covering 125 or skin. The outer covering 125 generallymay function as exterior cover that protects the internal components ofthe spacer 80 a from water intrusion, for example. In some embodiments,the outer covering 125 may be made from a flexible, acousticallytransparent material, which may be a plastic and/or elastomericmaterial, such as polyurethane. In some embodiments, the outer covering125 may have a thickness in a range of from about 0.5 mm to about 5 mm.In one particular embodiment, the outer covering 125 may have athickness of about 3 mm. As illustrated, the spacer 80 a may furtherhave channels 130 for the tension members 85. In the illustratedembodiment, the tension members 85 extend through the channels 130. Aspreviously mentioned, the tensions members 85 may be allowed to freelymove in the channels 130. As further illustrated, the spacer 80 a mayfurther comprise a compression member, such as central core 135. In theillustrated embodiment, the central core 135 is located in the middle ofthe spacer 80 a and has exterior surfaces 140 on either axial end of thespacer 80 a. The exterior surfaces 140 of the central core 135 mayengage adjoining exterior surfaces of adjacent spacers 80 whencompressed together in the operational configuration. The central core135 may comprise a material that can withstand the high axial loads thatcan be placed on the spacer 80 a in the operational configuration. Voidspaces in the spacer 80 a may be filled with a foamed material 140,which may comprise a foamed polyurethane or other suitable foam. Whilenot shown, the spacer 80 a may further comprise channels for conductorcables and cavities for geophysical sensors, transducers, and otherequipment used for geophysical data acquisition. Modifications may bemade to the spacer 80 a shown on FIG. 7 accommodate these additionalcomponents. For example, additional channels may be needed through thespacer 80 a for the conductor cables while additional cavities may beneeded for incorporation of other components. In addition, openingstoward pressure sensitive area (e.g., the outer covering 125) may beneed for geophysical sensors such as hydrophones to pick up pressuresignals in the water. Moreover, embodiments (not illustrated) mayreplace the central core 135 with a structure that provides a sealed andpressure-free space inside. In some embodiments, this structure may bean eggshell or cylindrically shaped structure. However, other suitableconfigurations for the structure may also be suitable.

FIGS. 8 and 9 illustrate a spacer 80 b having two layers of tensionmembers 85 that can be incorporated into the streamer segment 60 shownon FIGS. 2-6 in accordance with embodiments of the present invention.FIG. 8 shows a cut-away view of the spacer 80 b while FIG. 9 shows anend view of the spacer 80 b of FIG. 8. As best seen in FIG. 9, thespacer 80 b includes a plurality of outer channels 130 a and a pluralityof inner channels 130 b. The outer channels 130 a and the inner channels130 b may be arranged to form two concentric rings. Outer tensionsmembers 85 a and inner tensions members 85 b may be disposed through theouter channels 130 a and the inner channels 130 b, respectively.Accordingly, the spacer 80 b may comprise an outer annular layer of theouter tension members 85 a and an inner annular layer of the innertension members 85 b. As illustrated, the inner tension members 85 b mayhave a larger diameter than the inner tension members 85 a. In addition,channels (not shown) may be made in the spacer 80 b between the innerand outer channels 130 b, 130 a, for example, to allow a contact betweenthe water and the interior of the spacer 80 b or a contact path betweena hydrophone and the water. As illustrated by FIG. 9, the spacer 80 bmay further include one or more openings 160 for conductor cables (notshown) and a central opening 165 for the central core 135.

In accordance with present embodiments, the outer tension members 85 aand the inner tension members 85 b may be wound in opposite directionsas they extend through the streamer segment (e.g., streamer segment 60 ashown on FIGS. 2-6), as best seen on FIG. 10. If only one layer oftension members 85 is used, the streamer segment may rotate as tensionvaries along the tension members 85 due to strand stiffness and elasticdeformation. However, by using two concentric layers of tension members85 a, 85 b each wound in different directions, torsion balance can beachieved. In some embodiments, the tension members 85 a, 85 b maytransit between the inside and outside as they pass between spacers 80b. Because of this, the tension members 85 a, 85 b should notundesirably move inline during winching as they otherwise would. Theinner tension members 85 a may be slightly larger than the outer tensionmembers 85 to compensate for a smaller average radius from the center ofthe spacer 80 b. Tension in the tension members 85 a, 85 b should notlead to torsion because the effect of the two layers should be generallyopposite and equal.

FIG. 11 illustrates an assembly 170 that comprises a plurality ofstreamer segments 60 coupled end-end in accordance with embodiments ofthe present invention. As previously mentioned, the streamer segments 60may be coupled end-to-end and towed, for example, behind survey vessel10 (shown on FIG. 1). In some embodiments, the assembly 170 of streamersegments 60 may be used to form a streamer or a source cable. Thestreamer segments 60 may be a structure for a number of items, includingfeed lines, gas lines, optical and/or electrical signals, power,external devices, geophysical sensors, tension sensors, and geophysicalsources. In accordance with present invention, the streamer segments 60may have a variable stiffness, wherein the streamer segments 60 have anoperational configuration with an increased stiffness. Embodiments forincreasing the stiffness of the streamer segments 60 in the operationalconfiguration are discussed with respect to the preceding figures.

Accordingly, embodiments may include using a streamer having a variablestiffness in a marine survey. One of the many potential advantages isthat the streamer may be made stiffer than the previously used cables.For example, embodiments of the streamer may have a higher inlinestiffness which may lead to more accurate sensor positions due tosmaller and more predictable elongation and creep. By way of furtherexample, embodiments of the streamer may have higher bending stiffnesswhich may lead to lower drag, cross-flow, and noise. In addition,embodiments of the steamer may have higher rotational stiffness leadingto less motion and noise such that fewer sensors may be required todetect noise. Another potential advantage is that foamed materials maybe used to substantially occupy void volumes rather than the use ofoil/gels in typical streamer segments. Because foamed materials haveless density that oil/gels, embodiments may include use of more tensionmembers without undesirably increasing weight of the streamer segment.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Although individual embodiments arediscussed, the invention covers all combinations of all thoseembodiments. Furthermore, no limitations are intended to the details ofconstruction or design herein shown, other than as described in theclaims below. It is therefore evident that the particular illustrativeembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the presentinvention. All numbers and ranges disclosed above may vary by someamount. Whenever a numerical range with a lower limit and an upper limitis disclosed, any number and any included range falling within the rangeare specifically disclosed. Moreover, the indefinite articles “a” or“an,” as used in the claims, are defined herein to mean one or more thanone of the element that it introduces. Also, the terms in the claimshave their plain, ordinary meaning unless otherwise explicitly andclearly defined by the patentee. If there is any conflict in the usagesof a word or term in this specification and one or more patent or otherdocuments that may be incorporated herein by reference, the definitionsthat are consistent with this specification should be adopted for thepurposes of understanding this invention.

What is claimed:
 1. A geophysical survey method comprising: deploying acable into a body of water; changing the cable from a storableconfiguration to an operational configuration in the body of water,wherein the operational configuration has a higher bending stiffnessthan the storable configuration, wherein the changing the cable from thestorable configuration comprises applying tension to tension membersextending through a segment of the cable, the tension not being towingtension, wherein applying the tension to the tension members compressesspacers disposed in the segment of the cable causing the spacers toalign in a rigid line; and either (1) detecting a geophysical signalwith a sensor disposed on the cable or (2) activating a geophysicalsource on the cable.
 2. The method of claim 1 wherein the tensionmembers are disposed through channels in the spacers.
 3. The method ofclaim 1 wherein the tension applied to the tensions members is greaterthan towing tension applied to the tension members.
 4. The method ofclaim 1 wherein the spacers occupy at least 90% of an internal volume ofthe cable.
 5. The method of claim 1 wherein the cable comprises astreamer segment of a sensor streamer, wherein the streamer segment hasa length of from about 10 meters to about 150 meters.
 6. The method ofclaim 5 wherein the streamer has a length of about 200 meters or longer.7. The method of claim 1 wherein the sensor is disposed on the cable andthe method further comprises the step of detecting the geophysicalsignal with the sensor.
 8. The method of claim 1 wherein the cable inthe operable configuration has a bending stiffness of 700 Newton-squaremeters or greater over about 25 meters or more.
 9. The method of claim 1wherein the changing the cable from a storable configuration to anoperational configuration comprises moving an actuator plate outward totransfer force to tension members causing the tensioning members to moveaxially, wherein movement of the tension members cause a distal plate tocompress spacers into a rigid line, wherein the spacers are disposed inan internal volume of the cable, wherein a proximal plate and theactuator plate are located at a proximal end of the spacer, wherein thedistal plate is located at a distal end of the spacer, and wherein thetension members extend through the spacers along a length of the cable.10. A sensor streamer comprising: a plurality of streamer segments,wherein at least one of the streamer segments comprises: a jacket; aplurality of spacers disposed in the jacket along a length of the atleast one of the streamer segments; a proximal plate at a proximal endof the at least one of the streamer segments; a distal plate at a distalend of the at least one of the streamer segments, wherein the spacersare disposed between the proximal plate and the distal plate; anactuator plate at the proximal end; a plurality of tension members thatextend along a length of the at least one of the streamer segments,wherein the tension members extend from the proximal end to the distalend; and a tensioning actuator operable to transfer mechanical force tothe actuator plate through the plurality of tension members.
 11. Thesensor streamer of claim 10, wherein the at least one of the streamersegments has a length of from about 10 meters to about 150 meters. 12.The sensor streamer of claim 11, wherein the sensor streamer has alength of about 200 meters or longer, and wherein the plurality ofstreamer segments are connected end to end.
 13. The sensor streamer ofclaim 10, wherein the spacers comprise a foamed material, and whereinthe tension members comprise at least one member selected from the groupconsisting of a cable and a fiber rope.
 14. The sensor streamer of claim10, wherein the spacers occupy at least 90% of an internal volume of theat least one of the streamer segments.
 15. The sensor streamer of claim10, wherein the tensioning actuator comprises a linear drive operable togenerate motion in a straight line to move the actuator plate.
 16. Thesensor streamer of claim 10, wherein the tension members extendproximally beyond the proximal plate and the actuator plate, and whereinthe tension members extend from the proximal end through the spacers andthe distal plate.
 17. The sensor streamer of claim 16, wherein thetension members are translatable axially with respect to the actuatorplate, the proximal plate, the distal plate, and the spacers, andwherein mechanical stops at the proximal end and the distal end engagethe tension members.
 18. The sensor streamer of claim 16, wherein amechanical stop at the distal end is spring loaded with at least onespring between the mechanical stop and the distal plate such that, astension is applied to the tension members, the at least one spring iscompressed, and wherein a mechanical stop at the proximal end engagesthe actuator plate as the actuator plate translates.
 19. A sensorstreamer comprising: a plurality of streamer segments, wherein at leastone of the streamer segments comprises: a jacket; a plurality of spacersdisposed in the jacket along a length of the at least one of thestreamer segments; a proximal plate at a proximal end of the at leastone of the streamer segments; a distal plate at a distal end of the atleast one of the streamer segments, wherein the spacers are disposedbetween the proximal plate and the distal plate; an actuator plate atthe proximal end; a plurality of tension members that extend along alength of the at least one of the streamer segments, wherein the tensionmembers extend from the proximal end to the distal end, wherein thetension members are translatable axially with respect to the actuatorplate, the proximal plate, the distal plate, and the spacers, andwherein mechanical stops at the proximal end and the distal end engagethe tension members; and a tensioning actuator operable to transfermechanical force to the actuator plate.
 20. A sensor streamercomprising: a plurality of streamer segments, wherein at least one ofthe streamer segments comprises: a jacket; a plurality of spacersdisposed in the jacket along a length of the at least one of thestreamer segments; a proximal plate at a proximal end of the at leastone of the streamer segments; a distal plate at a distal end of the atleast one of the streamer segments, wherein the spacers are disposedbetween the proximal plate and the distal plate; an actuator plate atthe proximal end; a plurality of tension members that extend along alength of the at least one of the streamer segments, wherein the tensionmembers extend from the proximal end to the distal end, wherein thetension members extend proximally beyond the proximal plate and theactuator plate, wherein the tension members extend from the proximal endthrough the spacers and the distal plate, wherein a mechanical stop atthe distal end is spring loaded with at least one spring between themechanical stop and the distal plate such that, as tension is applied tothe tension members, the at least one spring is compressed, and whereina mechanical stop at the proximal end engages the actuator plate as theactuator plate translates; and a tensioning actuator operable totransfer mechanical force to the actuator plate.
 21. A geophysicalsurvey method comprising: deploying a cable into a body of water;changing the cable from a storable configuration to an operationalconfiguration in the body of water, wherein the operationalconfiguration has a higher bending stiffness than the storableconfiguration, wherein the changing the cable from a storableconfiguration to an operational configuration comprises moving anactuator plate outward to transfer force to tension members causing thetensioning members to move axially, wherein movement of the tensionmembers cause a distal plate to compress spacers into a rigid line,wherein the spacers are disposed in an internal volume of the cable,wherein a proximal plate and the actuator plate are located at aproximal end of the spacer, wherein the distal plate is located at adistal end of the spacer, and wherein the tension members extend throughthe spacers along a length of the cable; and either (1) detecting ageophysical signal with a sensor disposed on the cable or (2) activatinga geophysical source on the cable.